Many integrated circuit devices require stable oscillators to provide the reference clocks used in the system. Since virtually all integrated circuit devices operate in environments whose temperature fluctuates, such a reference clock should exhibit a stable oscillating frequency over a range of operating temperatures. Furthermore, it is desirable for a reference clock to exhibit a stable oscillating frequency over a range of operating voltages, and also not be affected by process variations which might arise during device manufacture.
Many applications necessitate that the device's power consumption be made as small as possible; for example, battery powered devices and/or RFID (radio frequency identification) tags benefit from a reduction in power consumption. In the case of battery powered devices, decreasing power consumption can increase the time required until battery recharging or replacement is required, and/or permit the use of a battery storing less power (which, typically, is a smaller, lighter and less costly battery).
In the case of an RFID device, power consumption and operating range vary inversely, so if power consumption decreases, the operating range increases.
The oscillator also should consume as little area as possible on silicon, at least in part to reduce the cost of manufacture of the device, to allow greater design flexibility, and to provide a more compact device.
Ring oscillators are one type of device suitable for use as clocks in integrated circuit devices. A ring oscillator can consist of a odd number of NOT gates which are connected in series such that the output of the last NOT gate is fed back to the input of the first NOT gate. Since each NOT gate cannot switch output state instantaneously, time is required for a signal to propagate from the input of the first NOT gate through the series of gates and thereby change the state at the output of the last NOT gate. This looping behaving contributes to the operating frequency of the ring oscillator.
It is known that the performance of ring oscillators is temperature dependent, as discussed by Datta et al. in the article “Analysis of a Ring Oscillator Based on Chip Thermal Sensor in 65 nm Technology” (2005).
Many consumer and professional electronic devices can be used at operating temperatures typically ranging from −40° C. to 125° C., particularly if those devices are portable. It is therefore likely that a device using a ring oscillator to provide a clock signal will experience temperature fluctuations, resulting in temperature-induced variations in the clock signal by that ring oscillator. Such clock signal variations could undesirably affect device performance, or require the inclusion of compensating components or logic in the device to cope with those variations.
Current solutions to this problem are not satisfactory. For example, one option, the RC oscillator, requires the use of a more-than-minimal silicon area.
Accordingly, there is a need for a ring oscillator whose performance is substantially independent of operating temperature.